SUBSURFACE RECHARGE SYSTEMS - From the MassHighway Storm Water Handbook for Highways and Bridges

 
 
SUBSURFACE RECHARGE SYSTEMS
From the MassHighway Storm Water Handbook for Highways and Bridges
http://www.mass.gov/dep/water/wastewater/stormwat.htm



Description:
Subsurface recharge systems may include trenches, beds, galleys, or dry wells. Such systems
have sufficient storage capacity so as to permit the gradual infiltration of runoff. Pollutant
removal is provided by filtration through the soil matrix. Pre-treatment is required to prevent
failure of infiltration systems due to sediment accumulation.

Subsurface systems (other than leaching catch basins or leaching basins will rarely be used in
the highway setting. These systems have historically had significant failure rates, and site
constraints often limit the effective use of infiltration.

Recharge BMPs should generally be designed as off-line systems. Separate Design Criteria
tables follow this table, for Recharge Trenches and Beds and Recharge Dry Wells and Galleys.



 GENERAL INFORMATION
 Applicable DEP Stormwater           Standards #3 and #4. If sufficient additional storage and appropriate
 Management Policy                   outlet structures are provided, recharge basins may also be used to
 Performance Standards               meet Standard #2.

 TSS Removal                         DEP Credit:         80%

                                     Estimated Range     >90%
                                     from Literature:

 TSS Removal                         Construction:         Moderate to high
                                     Maintenance:          High

 Potential Constraints to Use        • Depth to bedrock or other impermeable substratum
                                     • Depth to groundwater
                                     • Soils
                                     • Slope

 Other Considerations                • Recharge systems can provide high levels of treatment of other
                                       pollutants, in addition to TSS removal
                                     • High failure rates (particularly without sufficient pre-treatment);
                                       replacement/rehabilitation (with a cost about equal to initial
                                       construction) may be required
                                     • Frequent maintenance may be required

 Maintenance Requirements            • Inspect at least twice annually
                                     • Regular sediment removal from pre-treatment systems to prevent
                                       clogging
                                     • Rehabilitation in the event of failure due to clogging
                                     • Periodic removal of debris/trash from flow control structures

 Primary Design References           MA-DEP Stormwater Technical Handbook:
                                     http://www.mass.gov/dep/brp/stormwtr/stormpub.htm
                                     Schueler, 1987
                                     Young, et. al., 1996




                                  Subsurface Recharge Systems pg.1
                          Massachusetts Nonpoint Source Pollution Management
Recharge Trenches And Beds

DESIGN CRITERIA
DESIGN PARAMETER                        CRITERIA1
Contributing Drainage Area            < 2.0 ha (5 acres)

Storm frequency for design flow        Varies with system. Recharge systems are sized for recharging an
                                      annual volume, not an event storm.

Annual Recharge Volume                Compute annual recharge volume using methodology specified in
                                      MA-DEP Stormwater Technical Handbook (see Primary Design
                                      References), or use an alternative method conforming to accepted
                                      engineering practice.
Required Storage Volume               Compute storage volume using methodology specified in MA-DEP
                                      Stormwater Technical Handbook.

Design Recharge Rate                  Determine recharge rate based on soil texture/hydrologic group as
                                      specified in MA-DEP Stormwater Technical Handbook, confirmed
                                      by on-site field testing; or use an alternative method conforming to
                                      accepted engineering practice.

Design Safety Factor                  A safety factor is recommended to allow for the potential clogging of
                                      underground systems.

Maximum Draw-down Time                The system should be designed to drain the design storage volume in
                                      48 hours or less, using the design recharge rate times the applicable
                                      safety factor.
Maximum Depth of System               Depth of system shall be equal to or less than the depth permitting
                                      draw-down in the required time.
Stone Void Space                      When the void space in crushed stone is used for storage, the speci-
                                      fied stone should be uniformly sized. A porosity (volume of voids
                                      divided by total volume of bed) of 0.39 or less should be used for
                                      design.

Geotextile                            The crushed stone material must be isolated from adjacent in-situ
                                      soils by a geotextile fabric designed to prevent the migration of fine
                                      soil particles into the void spaces in the stone. Geotextile materials
                                      must meet applicable standard specifications, and must be selected
                                      based on an analysis of on-site soils conditions.

Provisions for Overflow or Bypass      An underground system should be designed as an “off-line” system,
                                      or otherwise provided with an overflow or by-pass to safely convey
                                      flows that exceed the system capacity.

Depth to Bedrock or Impermeable       Minimum 0.6 meters (2 feet) below bottom of system, unless
Stratum                               engineering analysis demonstrates that lesser separation is feasible.

Depth to seasonal high                Small systems: Minimum 0.6 meters (2 feet) below bottom of system.
groundwater                           Large systems: Groundwater mounding analysis may be required.

Pretreatment                          Pre-treatment system is required to provide TSS removal prior to
                                      discharge to an underground recharge system.

Velocity Dissipation at Inlet         Provide measures to dissipate velocity of flows into the device, to
                                      prevent erosion within the structure; generally, velocities < 0.61
                                      meters/second (2 fps) are recommended.




                                    Subsurface Recharge Systems pg.2
                           Massachusetts Nonpoint Source Pollution Management
Recharge Basin (Continued)

    DESIGN CRITERIA
    DESIGN PARAMETER                          CRITERIA1
    Setback from slab foundation            3.0 meters (10 feet)
    Setback from cellar foundation          6.1 meters (20 feet)
    Setback from slope >15%                 4.6 meters (15 feet) (top edge of system to top of slope), or as
                                            required for impoundment stability. Distance may need to be
                                            greater where potential for “break-out” and resulting slope
                                            instability may be a problem.
    Setback from on-site sewage             7.6 meters (25 feet) (or greater, if required under 310 CMR 15.000
    disposal system                         [Title 5])
    Setback from private well               30.5 meters (100 feet)
    30.5 meters (100 feet)                  Zone I radius; additional setback may be required depending on
                                            hydro-geologic conditions
    Setback from surface water supply       Zone A, and 30.5 meters (100 feet) from tributaries

    Construction of infiltration surface     The infiltration surface shall be constructed to preserve and
                                            enhance the capability of the soil to pass flows from the basin into
                                            the groundwater. Consider measures such as minimizing trafficking
                                            by heavy construction equipment

    Structural design loading for           If structural chambers are used to construct the bed or trench,
    chambers                                they should be designed for dead and live loads appropriate to
                                            their location. The minimum design load shall be H-20 loading.

    Inspection access                       Underground systems should be provided with access ports,
                                            man-ways, or observation wells to enable inspection of water
                                            levels within the system. At a minimum, provide two (2) ob-
                                            servation wells (152.40 mm (6-inch) diameter perforated PVC
                                            or HDPE risers) per trench or bed; for beds greater than 372.0
                                            square meters (4,000 square feet) in area, provide one (1) well for
                                            each 186.0 square meters (2,000 square feet) (minimum of three
                                            wells). The inspection port should be accessible at-grade (i.e. not
                                            buried).
    Access for maintenance, repair, and     Design shall consider accessibility to system, and capability to
    rehabilitation                          replace system components, to provide for eventual repair and
                                            rehabilitation of the system.
    Protection During Construction          Runoff from disturbed areas shall not be discharged to the
                                            recharge structure. The contributing site shall be completely
                                            stabilized, prior to placing the recharge structure in service.

    Other                                   Recharge trenches and beds should be “off-line” devices, with
                                            provisions for the bypassing or overflow of storms exceeding the
                                            storage capacity of the trench or bed.


1
 Several of the design criteria regarding setbacks from slopes, foundations, and other site features have been
adapted from the requirements for on-site sewage disposal systems described in Massachusetts Title 5 (310
CMR 15.000). However, storm water quantities and flow durations differ markedly from the hydraulic loadings
to septic systems. The design engineer should be aware of these differences, and may need to consider additional
setbacks to provide for slope stability, protect structures, and provide for the satisfactory performance of the
recharge system.



                                          Subsurface Recharge Systems pg.3
                                Massachusetts Nonpoint Source Pollution Management
Example of Recharge Trench




                        Subsurface Recharge Systems pg.4
               Massachusetts Nonpoint Source Pollution Management
Example of Recharge Bed




                      Subsurface Recharge Systems pg.5
              Massachusetts Nonpoint Source Pollution Management
Recharge Dry Wells And Galleys
DESIGN CRITERIA
DESIGN PARAMETER                       CRITERIA2
Contributing Drainage Area           Contributing area will be limited by the size of well or galley used.
                                     These devices are typically used for discharging roof top runoff, or
                                     small parking areas. Designer will need to relate size and number of
                                     units to the volume of runoff to be treated.
Storm frequency for design flow       Varies with system. Recharge systems are sized for recharging an
                                     annual volume, not a storm event.

Annual Recharge Volume               Compute annual recharge volume using methodology specified in
                                     MA-DEP Stormwater Technical Handbook (see Primary Design
                                     References), or use an alternative method conforming to accepted
                                     engineering practice.
Required Storage Volume              Compute storage volume using methodology specified in MA-DEP
                                     Stormwater Technical Handbook.

Design Recharge Rate                 Determine recharge rate based on soil texture/hydrologic group as
                                     specified in MA-DEP Stormwater Technical Handbook., confirmed
                                     by on-site field testing; or use an alternative method conforming to
                                     accepted engineering practice.

Design Safety Factor                 It is recommended that a safety factor be provided to allow for the
                                     potential clogging of underground systems.

Maximum Draw-down Time               The system should be designed to drain the design storage volume in
                                     48 hours or less, using the design recharge rate times the applicable
                                     safety factor.
Maximum Depth of System              Depth of system shall be equal to or less than the depth permitting
                                     draw-down in the required time.
Stone Void Space                     When the void space in crushed stone is used for storage, the speci-
                                     fied stone should be uniformly sized. A porosity (volume of voids
                                     divided by total volume of bed) of 0.39 or less should be used for
                                     design.

Geotextile                           The crushed stone material must be isolated from adjacent in-situ
                                     soils by a geotextile fabric designed to prevent the migration of fine
                                     soil particles into the void spaces in the stone. Geotextile materials
                                     shall meet applicable standard specifications, and must be selected
                                     based on an analysis of on-site soils conditions.

Provisions for Overflow or Bypass     An underground system should be designed as an “off-line” system,
                                     or otherwise provided with an overflow or by-pass to safely convey
                                     flows that exceed the system capacity.

Depth to Bedrock or Impermeable      Minimum 0.6 meter (2 feet) below bottom of system.
Stratum
Depth to seasonal high               Small systems: Minimum 0.6 meter (2 feet) below bottom of system.
groundwater                          Large systems: Groundwater mounding analysis may be required.

Pretreatment (roofs)                 Pre-treatment of residential roof runoff not required.
                                     Pre-treatment of commercial and industrial building roof runoff may
                                     need to be considered, on site-specific basis.
Pretreatment (other areas)           Pre-treatment system required; provide TSS removal rate specified by
                                     DEP prior to discharge to an underground recharge system.



                                   Subsurface Recharge Systems pg.6
                          Massachusetts Nonpoint Source Pollution Management
DESIGN CRITERIA
    DESIGN PARAMETER                         CRITERIA2
    Velocity Dissipation at Inlet          Provide measures to dissipate velocity of flows into the device, to
                                           prevent erosion within the structure; generally, velocities < 0.61
                                           meters/second (2 fps) are recommended.
    Setback from slab foundation           3.0 meters (10 feet)

    Setback from cellar foundation         6.1 meters (20 feet)
    Setback from slope >15%                50 feet (top edge of system to top of slope), or as required for
                                           impoundment stability. Distance may need to be greater where
                                           potential for “break-out” and resulting slope instability may be a
                                           problem.
    Setback from on-site sewage            7.6 meters (25 feet) (or greater, if required under 310 CMR 15.000
    disposal system                        [Title 5])

    Setback from private well              30.5 meters (100 feet)
    Setback from groundwater               Zone I radius; additional setback may be required depending on
    supply                                 hydro-geologic conditions

    Setback from surface water             Zone A, and 30.5 meters (100 feet) from tributaries
    supply
    Structural design loading              Structural components should be designed for dead and live loads
                                           appropriate to their location. The minimum design load shall be
                                           H-20 loading.
    Inspection access                      Each well or galley unit should be provided with an access port,
                                           man-way, or observation well to enable inspection of water levels
                                           within the system. The inspection port should be accessible
                                           at-grade (i.e. not buried).

    Maintenance access                     If inspection port does not provide access, additional manhole
                                           access should be provided to each well or galley chamber.

    Protection During Construction         Runoff from disturbed areas shall not be discharged to the recharge
                                           structure. The contributing site shall be completely stabilized, prior
                                           to placing the recharge structure in service.
    Other                                  Recharge dry wells and galleys should be “off-line” devices, with
                                           provisions for bypassing or overflow of storms exceeding the design
                                           capacity of the devices.



2
 Several of the design criteria regarding setbacks from slopes, foundations, and other site features have been
adapted from the requirements for on-site sewage disposal systems described in Massachusetts Title 5 (310
CMR 15.000). However, storm water quantities and flow durations differ markedly from the hydraulic loadings
to septic systems. The design engineer should be aware of these differences, and may need to consider additional
setbacks to provide for slope stability, protect structures, and provide for the satisfactory performance of the
recharge system.




                                        Subsurface Recharge Systems pg.7
                                Massachusetts Nonpoint Source Pollution Management
Example of Recharge Galley




                        Subsurface Recharge Systems pg.8
               Massachusetts Nonpoint Source Pollution Management
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